1. Field of the Invention
The present invention relates to a thermal printer wherein a thermal print head mounted on a carriage is moved along a printing line on a recording medium, for thermal printing on the medium with a print ribbon having a thermally fusible ink layer.
2. Discussion of the Prior Art
In a known thermal printer, a drive for reciprocating the carriage is utilized to feed a print ribbon from a supply spool to a take-up spool. The carriage incorporates a mechanism for converting a linear movement of the carriage into a rotary movement of the take-up spool. For example, the carriage rotatably supports a take-up spool shaft engageable with the take-up spool, and a suitable rotating member such as a pinion. The pinion is adapted to engage an elongate stationary toothed belt or rack which extends in the direction of movement of the carriage, so that the pinion is rotated as the carriage is moved for printing. The carriage further has a power transmitting system for transmitting a rotary motion of the pinion to the take-up spool shaft and to the take-up spool for winding the used length of the print ribbon which has passed the print head. The power transmitting mechanism incorporates a frictional coupling device which has a first friction member rotatable with the take-up spool shaft, and a second friction member rotatable with the pinion, so that the rotation of the pinion is imparted to the take-up spool shaft by means of frictional engagement of the first and second friction members of the coupling device. The frictional coupling device permits a slip between the first and second friction members, so as to prevent the take-up spool from rotating when a rotational resistance applied to the take-up spool exceeds a given upper limit (a friction force between the friction members).
The frictional coupling device is provided in order to adjust the rotating speed of the take-up spool, as a function of the length of the print ribbon that has been wound on the take-up spool. More specifically, the effective diameter of the take-up spool, that is, the outside diameter of the ribbon wound on the take-up spool increases as the ribbon is fed from the supply spool to the take-up spool. Accordingly, the rate at which the print ribbon is wound on the take-up spool is gradually increased. Since the ribbon winding or take-up speed must be substantially equal to the speed of the carriage movement, the rotating speed of the take-up spool must be reduced as the diameter of the print ribbon wound on the take-up spool is increased.
With the frictional coupling device incorporated in the power transmitting mechanism between the pinion and the take-up spool shaft, there arises a slip between the first and second friction members while a frictional resistance applied to the take-up spool exceeds the friction force between the two friction members. The rotational resistance may exceed the friction force due to the higher winding speed of the take-up spool than the carriage moving speed. Thus, the amount of slip between the first and second friction members of the coupling device increases with an increasing diameter of the ribbon wound on the take-up spool increases.
However, the use of such a frictional coupling device complicates the power transmitting mechanism incorporated in the carriage, and results in increasing the size and weight of the carriage assembly as a whole. This necessitates a carriage drive that has a comparatively large capacity, and consequently leads to increased energy consumption of the printer.
The above tendency toward increased size and weight of the carriage due to the use of a frictional coupling device is created, since the carriage incorporates a rotating member such as a pinion which is kept in motion while engaging a longitudinal fixed member such as a rack, and further incorporates a frictional coupling device for connection of the rotating member to the take-up spool shaft in such a manner as to prevent the take-up spool from rotating with the rotating member. The tendency is remarkable particularly where the friction members of the coupling device have a relatively large surface of frictional contact with each other, for providing a consistent friction force therebetween, and thereby assuring a consistent drive force to rotate the take-up spool with sufficient operating stability. The increase in the size and weight of the carriage, and the need of a larger capacity of the carriage drive means, are contrary to the recent requirement for reduced size and energy consumption of the thermal printer, for lowering the manufacturing and operating costs of the printer.